Default Value

This is pretty tricky, but sometimes you can accomplish it when the conditions are correct (and your requirements not too finnicky).  Caveat that you need to *really* know what you're doing in terms of SAIL / expression coding, or else you'll probably get lost.

Generally what you want here isn't as much "show default value", but more like, "when a value for dropdown A is selected that causes dropdown B to only have one choice, save that choice for value B automatically".  I make this distinction because even if you manage to trick Dropdown B into "showing" the only value, it won't actually persist that value into the desired target unless you take extra action.

The easiest place in which to accomplish this is probably the SaveInto for Dropdown A - but that still depends on some conditions, like the source of the data for the choices for B (is it querying? reading a constant? hardcoded in the form? etc?).

You have to pass data conditionally based on the first drop down value through parent Interface/Process model.